The invention relates to a method of determining the concentration of electro-chemically convertible substances in a solution by, means of an analyzer which includes a test electrode.
Determining the concentration of dissolved substances, particularly in the presence of foreign substances causing interference, is important in many fields. A representative example of this is the determination of glucose in body fluids such as blood. Such a determination is especially necessary for diabetics.
Diabetic therapy can be greatly improved if patients are continuously, i.e., at a constant basal rate, supplied with insulin, and if additional doses of insulin are called for before meals. In keeping with this principle, implantable pumps which are connected to a container filled with a supply of insulin have already been developed. The pumps are operated by a programmable control unit. However, the objective here is a closed control system which requires a sensor for determining the concentration of glucose in the patient's blood.
Implantable electro-catalytic sugar sensors for determining the concentration of glucose are already in existence. Such sensors have a test electrode, of platinum for example, which is located in front of a membrane which is placed so that it determines the rate of diffusion (see U.S. Pat. No. 4,366,033). In order to determine the concentration of glucose, a potential profile is impressed on the test electrode using a potentiostat, whereby two levels are specified: a positive and a negative potential. In the case of the positive potential, the test electrode is regenerated; i.e., the reaction products on the electrode are oxidized. If the test electrode reaches the negative potential the current is integrated. The charge determined here is the test signal which is allocated to the glucose concentration. However, in the case of the implantable electro-catalytic glucose sensors, difficulties arise in suppressing the influence of interfering factors such as urea and amino acids which are components of the body fluid.